Looks like you are using one of the lesser-used features of CellProfiler.
As far as I can tell, the post which started this thread 10 (!) years ago refers to a feature that no longer exists in CellProfiler. The histogram data tool does not seem to allow combining results from multiple image sets anymore. I'm afraid you will need to use a standard spreadsheet program (Excel or Google docs) to accomplish this task.
Alternately, I think you can accomplish what you want in CellProfiler Analyst but I won't guarantee it, since it's been quite a long time since I've tested it. Using CPA requires that you create a properties file in your CellProfiler pipeline, in the Export to Database module. It's powerful software but takes a few steps to get going. It's a good investment to learn how to get your data into it, but a spreadsheet program would be simpler to accomplish your immediate goals.